IRE Ablation Systems and Protocols Using a Basket Catheter

ABSTRACT

A medical apparatus includes a probe, including an insertion tube configured for insertion into a body cavity of a patient and a basket assembly, which has a proximal end that is connected distally to the insertion tube and includes a plurality of resilient spines, which are configured to bow radially outward around a longitudinal axis of the basket assembly and are conjoined at a distal end of the basket assembly. A plurality of electrodes are configured to contact tissue in the body cavity and include radial electrodes disposed on the spines and an axial electrode disposed on the longitudinal axis of the basket assembly. An electrical signal generator is configured to apply to the electrodes, including the axial electrode, pulses having an amplitude sufficient to cause irreversible electroporation (IRE) in the tissue contacted by the electrodes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application 63/076,614, filed Sep. 10, 2020, whose disclosure is incorporated herein by reference as if set forth in full into this application.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical equipment and procedures, and particularly to apparatus and methods for ablating tissue within the body.

BACKGROUND

Irreversible electroporation (IRE) is a soft tissue ablation technique that applies short pulses of strong electrical fields to create permanent and hence lethal nanopores in the cell membrane, thus disrupting the cellular homeostasis (internal physical and chemical conditions). Cell death following IRE results from apoptosis (programmed cell death) and not necrosis (cell injury, which results in the destruction of a cell through the action of its own enzymes) as in other thermal and radiation-based ablation techniques. IRE is commonly used in tumor ablation in regions where precision and conservation of the extracellular matrix, blood flow and nerves are of importance.

Some electroporation techniques use basket catheters, in which electrodes are mounted on flexible spines at the distal end of the catheter, and the spines bend outward to form a basket-like shape and contact tissue within a body cavity. For example, U.S. Patent Application Publication 2020/0289197 describes devices and methods for electroporation ablation therapy, with the device including a set of splines coupled to a catheter for medical ablation therapy. Each spline of the set of splines may include a set of electrodes formed on that spline. The set of splines may be configured for translation to transition between a first configuration and a second configuration.

SUMMARY

Embodiments of the present invention that are described hereinbelow provide improved systems and methods for IRE.

There is therefore provided, in accordance with an embodiment of the invention, a medical apparatus, which includes a probe, including an insertion tube configured for insertion into a body cavity of a patient and a basket assembly having a proximal end that is connected distally to the insertion tube and including a plurality of resilient spines, which are configured to bow radially outward around a longitudinal axis of the basket assembly and are conjoined at a distal end of the basket assembly. A plurality of electrodes, which are configured to contact tissue in the body cavity, include radial electrodes disposed on the spines and an axial electrode disposed on the longitudinal axis of the basket assembly. An electrical signal generator is configured to apply to the electrodes, including the axial electrode, pulses having an amplitude sufficient to cause irreversible electroporation (IRE) in the tissue contacted by the electrodes.

In the pictured embodiments, the spines have respective proximal and distal tips, wherein the proximal tips of the spines are coupled together at a at a proximal end of the basket assembly, and the distal tips of the spines are coupled together at a at a distal end of the basket assembly, and the spines bow radially outward when the basket assembly is deployed in the body cavity, whereby the radial electrodes contact the tissue in the body cavity. In one embodiment, the basket assembly comprises a stable collapsed state, and wherein the apparatus includes a puller attached to the distal end of the basket assembly and slidably disposed within the insertion tube, so that the spines bow radially outward in response to pulling the puller in a proximal direction through the insertion tube. Additionally or alternatively, the insertion tube includes a flexible catheter configured for insertion into a chamber of a heart of the patient, and the electrodes are configured to contact and apply the electrical signals to myocardial tissue within the chamber.

In one embodiment, the radial electrodes include a single respective radial electrode on each of the spines. In another embodiment, the radial electrodes include multiple radial electrodes on each of the spines in respective locations that are longitudinally staggered among the spines.

Typically, the pulses applied by the electrical signal generator include a sequence of the pulses having an amplitude of at least approximately 200 V, and a duration of each of the pulses is less than approximately 20 μs. In a disclosed embodiment, the sequence of the pulses includes biphasic pairs of the pulses, wherein each pair includes a positive pulse and a negative pulse.

In a disclosed embodiment, the electrical signal generator is configured to apply the pulses in a bipolar mode between one or more of the radial electrodes and the axial electrode. Alternatively or additionally, the electrical signal generator is configured to apply the pulses in a unipolar mode between one or more of the electrodes on the basket assembly, including the axial electrode, and a common electrode that is separate from the probe.

In some embodiments, the electrical signal generator is configured to apply the pulses between different pairs of the electrodes sequentially in accordance with a predefined protocol. In one embodiment, in accordance with the predefined protocol, the pulses are applied in a bipolar mode between each of the radial electrodes and the axial electrode. The pulses may be applied simultaneously between two or more of the radial electrodes and the axial electrode. Additionally or alternatively, in accordance with the predefined protocol, the pulses are applied in a bipolar mode between multiple pairs of the radial electrodes, each pair including a first radial electrode on a first one of the spines and a second radial electrode on a second of the spines, which is not adjacent to the first one or the spines. Further additionally or alternatively, in accordance with the predefined protocol, the pulses are applied to the radial electrodes on different ones of the spines in a predefined sequence in which the pulses are not applied in immediate succession to the radial electrodes on mutually adjacent spines.

In another embodiment, the radial electrodes include at least first and second radial electrodes on each of the spines, and in accordance with the predefined protocol, the pulses are applied to only one of the first and second radial electrodes on each of the spines.

There is also provided, in accordance with an embodiment of the invention, a method for medical treatment, which includes providing a basket assembly for insertion into a body cavity of a patient, the basket assembly including a plurality of resilient spines, which are configured to bow radially outward around a longitudinal axis of the basket assembly and are conjoined at a distal end of the basket assembly. A plurality of electrodes, which are configured to contact tissue in the body cavity, include radial electrodes disposed on the spines and an axial electrode disposed on the longitudinal axis of the basket assembly. Pulses are applied to the electrodes, including the axial electrode, with an amplitude sufficient to cause irreversible electroporation (IRE) in the tissue contacted by the electrodes.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic pictorial illustration of a system used in an IRE ablation procedure, in accordance with embodiments of the invention;

FIG. 2 is a schematic pictorial illustration of the distal end of a basket catheter used in IRE, in accordance with an embodiment of the invention;

FIG. 3 is a flow chart, which schematically illustrates a method for IRE, in accordance with an embodiment of the invention; and

FIG. 4 is a schematic pictorial illustration of the distal end of a basket catheter used in IRE, in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

IRE is a predominantly non-thermal process, which causes an increase of the tissue temperature by, at most, a few degrees for a few seconds. It thus differs from RF (radio frequency) ablation, which raises the tissue temperature by between 20 and 70° C. and destroys cells through heating. IRE typically utilizes biphasic pulses, i.e., combinations of positive and negative electrical pulses, in order to avoid muscle contraction due to a cumulative DC voltage. The pulses are applied, for example, between two bipolar electrodes of a catheter.

Because IRE is an electrical, rather than a thermal, process, different cells will respond differently to the electric field generated by the IRE depending, for example, on the different shapes, sizes, and orientations of the cells. The effectiveness of IRE ablation may therefore depend not only on the amplitudes of the electrical pulses that are applied to the tissue, but also on the geometrical directions. To ensure effective ablation, it is therefore desirable that electric fields be applied to the target tissue along multiple different directions. This sort of multi-directional approach can be difficult or impossible to implement using a conventional focal catheter, however, or even using basket catheters that are known in the art.

Embodiments of the present invention that are described herein provide basket catheters with a three-dimensional arrangement of electrodes that can be used to address this problem. Specifically, the disclosed embodiments provide a probe, such as a catheter, comprising an insertion tube for insertion into a body cavity, such as a chamber of the heart, and a basket assembly, whose proximal end is connected distally to the insertion tube. The basket assembly comprises resilient spines, which bow radially outward around the axis of the basket assembly and are conjoined at the distal end of the basket assembly. One or more radial electrodes are disposed on each of the spines, and an axial electrode is disposed at the distal end of the basket assembly, i.e., on the axis of the basket assembly at its distal tip.

An electrical signal generator applies IRE pulses to the electrodes, including the axial electrode. (The term “IRE pulses” is used in the present description and in the claims to mean electrical pulses having an amplitude sufficient to cause IRE in the tissue contacted by the electrodes.) The presence of the axial electrode makes it possible for the pulses to be applied in a bipolar mode in multiple different directions, both between different pairs of the radial electrodes and between the radial electrodes and the axial electrode. Additionally or alternatively, the electrical signal generator can apply the pulses in a unipolar mode between any of the radial electrodes and/or the axial electrode and a common electrode, such as a back patch, that is separate from the probe. Thus, using this sort of basket catheter with an axial (tip) electrode, IRE pulses can be applied to the target tissue in multiple different directions, while the basket is held stationary against the tissue.

To ablate a particular region of tissue effectively using this sort of catheter, the IRE pulses should be applied sequentially to different electrodes and pairs of electrodes on the catheter, such that the electric field reaches all areas of the tissue in multiple different field orientations. It is also desirable that the sequence of actuation of the electrodes by the IRE pulses be chosen so that the electrical power and residual heat are dissipated evenly between the electrodes. Although this sort of selection can be performed manually by the practitioner carrying out the ablation procedure, it is time-consuming, tedious, and error prone.

Therefore, some embodiments of the present invention provide predefined IRE ablation protocols that the practitioner can choose. Each protocol designates a sequence of electrodes or pairs of electrodes to which IRE pulses are to be applied in order to cover the region to be ablated with all the desired electric field orientations. The protocols may also specify the IRE pulse parameters for each electrode or pair of electrodes in the sequence. Both unipolar and bipolar ablation protocols may be defined in this manner. For effective dissipation of electrical and thermal energy, the protocols may desirably specify the sequence of electrodes such that the IRE pulses are not applied in immediate succession to radial electrodes on mutually adjacent spines.

In bipolar protocols, for example, IRE pulses are applied in a bipolar mode between pairs of the radial electrodes and between each of the radial electrodes and the axial electrode. In some bipolar protocols, the pulses are applied simultaneously between two or more of the radial electrodes and the axial electrode. The IRE pulses may be applied between pairs of the radial electrodes on different spines that are not mutually adjacent. When the basket assembly comprises two or more radial electrodes on each of the spines, the protocol may specify that the IRE pulses be applied to only a subset of the radial electrodes on each spine, such as the most distal radial electrode or the most proximal.

System Description

FIG. 1 is a schematic pictorial illustration of a system 20 used in an IRE ablation procedure, in accordance with an embodiment of the invention. Elements of system 20 may be based on components of the CARTO® system, produced by Biosense Webster, Inc. (Irvine, Calif.).

A physician 30 navigates a catheter 22 through the vascular system of a patient 28 into a chamber of a heart 26 of the patient, and then deploys a basket assembly 40 (shown in detail in FIG. 2) at the distal end of the catheter. The proximal end of basket assembly 40 is connected to the distal end of an insertion tube 25, which physician 30 steers using a manipulator 32 near the proximal end of catheter 22. Basket assembly 40 is inserted in a collapsed configuration through a sheath 23, which passes through the vascular system of patient 28 into the heart chamber where the IRE procedure is to be performed, and is then deployed from the sheath and allowed to expand within the chamber. Catheter 22 is connected at its proximal end to a control console 24. A display 27 on console 24 may present a map 31 or other image of the heart chamber with an icon showing the location of basket assembly 40 in order to assist physician 30 in positioning the basket assembly at the target location for the IRE ablation procedure.

Once basket assembly 40 is properly deployed and positioned in heart 26, physician 30 actuates an electrical signal generator 38 in console 24 to apply sequences of IRE pulses to the electrodes on the basket assembly, under the control of a processor 36. The IRE pulses are typically applied in a bipolar mode, between pairs of the electrodes on basket assembly 40. Additionally or alternatively, the pulses may be applied between the electrodes on basket assembly 40 and a separate common electrode, for example a conductive back patch 42, which is applied to the patient's skin.

To perform IRE, electrical signal generator 38 typically applies sequences of pulses to the electrodes with an amplitude of at least approximately 200 V, while the duration of each pulse is less than approximately 20 μs. The sequences of the pulses comprise biphasic pairs of pulses, meaning that each pair comprises a positive pulse and a negative pulse. (The terms “positive” and “negative” are used arbitrarily to indicate the opposing electrical polarities of the pulses.) An electrical signal generator capable of outputting this sort of IRE pulse sequences, while switching rapidly between different pairs of electrodes, is described, for example, in U.S. patent application Ser. No. 16/701,989, filed Dec. 3, 2019, whose disclosure is incorporated herein by reference.

Processor 36 directs electrical signal generator 38 to switch among the electrodes and apply the pulses in accordance with a predefined protocol, such as one or more of the protocols described further hereinbelow. Physician 30 may select the protocol to apply using controls on console 24. For example, the physician or an assistant may use a touch screen functionality of display 27 on console to interact with processor 36. Alternatively or additionally, the physician or an assistant may operate the controls in order to select the pulse parameters and electrodes manually.

Typically, catheter 22 comprises one or more position sensors (not shown in the figures), which output position signals that are indicative of the position (location and orientation) of basket assembly 40. For example, basket assembly 40 may incorporates one or more magnetic sensors, which output electrical signals in response to an applied magnetic field. Processor 36 receives and processes the signals in order to find the location and orientation coordinates of basket assembly 40, using techniques that are known in the art and are implemented, for example, in the above-mentioned Carto system. Alternatively or additionally, system 20 may apply other position-sensing technologies in order to find the coordinates of basket assembly 40. For example, processor 36 may sense the impedances between the electrodes on basket assembly 40 and body-surface electrodes 39, which are applied to the chest of patient 28, and may convert the impedances into location coordinates using techniques that are likewise known in the art. In any case, processor 41 uses the coordinates in displaying the location of basket assembly 40 on map 31.

Alternatively, catheter 22 and the IRE ablation protocols that are described herein may be used without the benefit of position sensing. In such embodiments, for example, fluoroscopy and/or other imaging techniques may be used to ascertain the location of basket assembly 40 in heart 26.

The system configuration that is shown in FIG. 1 is presented by way of example for conceptual clarity in understanding the operation of embodiments of the present invention. For the sake of simplicity, FIG. 1 shows only the elements of system 20 that are specifically related to the disclosed techniques. The remaining elements of the system will be apparent to those skilled in the art, who will likewise understand that the principles of the present invention may be implemented in other medical therapeutic systems, using other components. All such alternative implementations are considered to be within the scope of the present invention.

Basket Assemblies and Ire Protocols

FIG. 2 is a schematic pictorial illustration of basket assembly 40, in accordance with an embodiment of the invention. Basket assembly 40 comprises multiple resilient spines 44, with one radial electrode 48 a, 48 b, 48 c, 48 d, 48 e, 48 f, 48 g, 48 h disposed on each of the spines. (These electrodes are referred to collectively as radial electrodes 48.) An axial electrode 50 is disposed at the tip of the basket, i.e., at the distal end of the basket assembly. Although basket assembly 40 is shown as comprising eight spines 44, in alternative embodiments the basket assembly may comprise a larger or smaller number of spines.

Spines 44 typically comprise a suitable, resilient metal or plastic material, for example. The proximal tips of spines 44 are coupled together at a at the proximal end of the basket assembly, where the basket assembly connects to the distal end of insertion tube 25. The distal tips of spines 44 are likewise coupled together (using suitable joining or coupling technologies) at the distal end of the basket assembly. The spines bow radially outward when basket assembly 40 is deployed from sheath 23 into the heart chamber. Physician 30 then manipulates catheter 22 so that electrodes 48 and 50 contact the myocardial tissue at the target location in the heart chamber.

Basket assembly 40 may comprise other components, as well (not shown in the figures), such as ultrasound transducers, contact force sensors, and temperature sensors. Electrodes 48 and 50, as well as these other components, are connected to wires (not shown) running through insertion tube 25 to the proximal end of catheter 22, where they connect to appropriate circuitry in console 24. Further details of the construction of spines 44, including mechanical and electrical attachment of electrodes 48 and 50 to basket assembly 40, are presented in the above-mentioned U.S. Provisional Patent Application 63/076,614. The surface mounted electrodes 48 a, 48 b, 48 c . . . 48 h extend beyond the surface of the spines 44 (i.e., bulging beyond the surface boundary of the spine) to ensure that the respective electrodes 48 a-48 h are in sufficient tissue contact. Alternatively, other designs of the electrodes and spines may be used, as will be apparent to those skilled in the art after reading the present description.

Various mechanisms can be used to collapse basket assembly 40 as it passes through sheath 23 and to cause spines 44 to bow radially outward when deployed from the sheath, so as to assume the expanded state that is shown in FIG. 2. In the present example, basket assembly 40 has a stable collapsed state, in which spines 44 straighten along the axial direction (i.e., parallel to the longitudinal axis of insertion tube 25). A puller 46, such as a metal or plastic rod or wire, is attached to the distal end of basket assembly 40, where axial electrode 50 is located and spines 44 are conjoined. Puller 46 is slidably disposed within insertion tube 25. To deploy the basket assembly, puller 46 is pulled in the proximal direction through the insertion tube, thus causing spines 44 to bow radially outward. It is noted that while electrode 50 is illustrated in one embodiment as being a dome shaped electrode, it is within the scope of the claimed invention to utilize a cylindrical electrode 51 instead of the dome shaped electrode 50 as well as both dome electrode 50 and cylindrical electrode 51.

FIG. 3 is a flow chart that schematically illustrates a method for IRE, in accordance with an embodiment of the invention. The method illustrated in the figure implements a protocol for bipolar ablation, in which IRE pulses are applied between different pairs of electrodes 48 and 50. For the sake of convenience, radial electrodes 48 a, 48 c, 48 e and 48 g are referred to as the “odd” electrodes in this context, while radial electrodes 48 b, 48 d, 48 f and 48 h are referred to as the “even” electrodes.

In an initial radial ablation step 52, electrical signal generator 38 applies IRE pulses in alternation between pairs of the odd radial electrodes, for example between electrodes 48 a and 48 c, then between electrodes 48 c and 48 e, then between electrodes 48 e and 48 g, and finally between electrodes 48 g and 48 a. In a second radial ablation step 54, electrical signal generator 38 applies IRE pulses in alternation between pairs of the even radial electrodes, for example between electrodes 48 b and 48 d, then between electrodes 48 d and 48 f, then between electrodes 48 f and 48 h, and finally between electrodes 48 h and 48 b.

After completing the radial ablation steps, electrical signal generator 38 applies IRE pulses between the odd radial electrodes 48 and axial electrode 50, at an initial axial ablation step 56. Thus, pulses are applied between electrodes 48 a and 50, then between electrodes 48 c and 50, then between electrodes 48 e and 50, and finally between electrodes 48 g and 50. In a second axial ablation step 58, electrical signal generator 38 applies IRE pulses between electrodes 48 b and 50, then between electrodes 48 d and 50, then between electrodes 48 f and 50, and finally between electrodes 48 h and 50. Alternatively, in steps 56 and 58, IRE pulses may be applied simultaneously between two or more of radial electrodes 48 and axial electrode 50.

The above order of actuation of the electrodes is described by way of example. Alternative protocols, with different orders of electrode actuation, will be apparent to those skilled in the art after reading the present description and are considered to be within the scope of the present invention.

In an alternative protocol, electrical signal generator 38 applies IRE pulses to electrodes 48 and 50 in a unipolar mode, for example between electrodes 48 and 50 and back patch 42. The electrodes to be actuated are chosen depending on the area of the myocardial tissue that is to be ablated. The electrodes may be actuated either individually, or simultaneously in groups of two or more, or all at once. Alternatively, the unipolar IRE pulses may be applied between electrodes 48 and 50 on basket assembly 40 and a common electrode inside the body, for example an electrode on another catheter, as long as this common internal electrode is sufficiently large to convey the IRE power.

As noted earlier, the IRE pulses applied to electrodes 48 and 50 typically have an amplitude of at least approximately 200 V, and a duration of each of the pulses is less than approximately 20 μs. In one embodiment, each selected set of electrodes is energized in bursts, wherein each burst comprises a sequence of pulses, for example ten biphasic pulses of amplitude 1000 V and pulse duration 2 μs, with a delay of 5 ms from pulse to pulse. Twenty bursts of this sort may be applied to each set of the electrodes. Alternatively, smaller or larger numbers of bursts may be applied, and different pulse parameters may be defined.

FIG. 4 is a schematic pictorial illustration of a basket assembly 60 at the distal end of a catheter used in IRE, in accordance with another embodiment of the invention. Basket assembly 60 is similar in construction and functionality to basket assembly 40, as described above, except that multiple radial electrodes are disposed on each spine 44, including proximal radial electrodes 62 a, 62 b, . . . , 62 h and distal radial electrodes 64 a, 64 b, . . . , 64 h. In the pictured embodiment, the locations of the radial electrodes along spines 44 are longitudinally staggered, i.e., different distal radial electrodes, such as electrodes 64 a and 64 b, are located at different distances from the distal ends of the respective spines 44 on which they are mounted; and proximal radial electrodes 62 a and 62 b are similarly mounted at different distances from the distal ends of the spines. This sort of staggering can be useful in varying the locations and directions across which IRE pulses are applied to the tissue with which basket assembly 60 is in contact. Alternatively, the basket assembly may comprise a larger number of radial electrodes, which may be staggered or uniformly positioned along the respective spines.

The same sorts of protocols, including both bipolar and unipolar protocols, may be applied in actuating the electrodes on basket assembly 60 as were described above with respect to basket assembly 40. The protocols may be modified to include actuation of all of radial electrodes 62 a, 62 b, . . . , 62 h and 64 a, 64 b, . . . , 64 h. Alternatively, only a subset of the radial electrodes may be actuated, depending on the therapeutic plan and contact between the electrodes and the target tissue. For example, IRE pulses may be applied in a bipolar mode only between distal radial electrodes 64 a, 64 b, . . . , 64 h and axial electrode 50.

It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. 

1. A medical apparatus, comprising: a probe, comprising: an insertion tube configured for insertion into a body cavity of a patient; a basket assembly having a proximal end that is connected distally to the insertion tube and comprising a plurality of resilient spines, which are configured to bow radially outward around a longitudinal axis of the basket assembly and are conjoined at a distal end of the basket assembly; and a plurality of electrodes, which are configured to contact tissue in the body cavity and comprise radial electrodes disposed on the spines about the longitudinal axis and an axial electrode disposed on the longitudinal axis of the basket assembly; an electrical signal generator configured to apply to the electrodes, including the axial electrode, pulses having an amplitude sufficient to cause irreversible electroporation (IRE) in the tissue contacted by the electrodes.
 2. The apparatus according to claim 1, wherein the spines comprise respective proximal and distal tips, wherein the proximal tips of the spines are joined proximate a proximal end of the basket assembly, and the distal tips of the spines are joined to a distal end of the basket assembly, and the spines bow radially outward away from the longitudinal axis when the basket assembly is deployed in the body cavity so that the radial electrodes contact the tissue in the body cavity.
 3. The apparatus according to claim 2, wherein the basket assembly comprises a stable collapsed state, and wherein the apparatus comprises a puller attached to the distal end of the basket assembly and slidably disposed within the insertion tube, so that the spines bow radially outward in response to pulling the puller in a proximal direction through the insertion tube.
 4. The apparatus according to claim 1, wherein the insertion tube comprises a flexible catheter configured for insertion into a chamber of a heart of the patient, and the electrodes are configured to contact and apply the electrical signals to myocardial tissue within the chamber.
 5. The apparatus according to claim 1, wherein the radial electrodes comprise a single respective radial electrode on each of the spines.
 6. The apparatus according to claim 1, wherein the radial electrodes comprise multiple radial electrodes on each of the spines in respective locations that are longitudinally staggered among the spines.
 7. The apparatus according to claim 1, wherein the pulses applied by the electrical signal generator comprise a sequence of the pulses having an amplitude of at least approximately 200 V, and a duration of each of the pulses is less than approximately 20 μs.
 8. The apparatus according to claim 7, wherein the sequence of the pulses comprises biphasic pairs of the pulses, wherein each pair comprises a positive pulse and a negative pulse.
 9. The apparatus according to claim 1, wherein the electrical signal generator is configured to apply the pulses in a bipolar mode between one or more of the radial electrodes and the axial electrode.
 10. The apparatus according to claim 1, wherein the electrical signal generator is configured to apply the pulses in a unipolar mode between one or more of the electrodes on the basket assembly, including the axial electrode, and a common electrode that is separate from the probe.
 11. The apparatus according to claim 1, wherein the electrical signal generator is configured to apply the pulses between different pairs of the electrodes sequentially in accordance with a predefined protocol.
 12. The apparatus according to claim 11, wherein in accordance with the predefined protocol, the pulses are applied in a bipolar mode between each of the radial electrodes and the axial electrode.
 13. The apparatus according to claim 12, wherein in accordance with the predefined protocol, the pulses are applied simultaneously between two or more of the radial electrodes and the axial electrode.
 14. The apparatus according to claim 11, wherein in accordance with the predefined protocol, the pulses are applied in a bipolar mode between multiple pairs of the radial electrodes, each pair including a first radial electrode on a first one of the spines and a second radial electrode on a second of the spines, which is not adjacent to the first one or the spines.
 15. The apparatus according to claim 11, wherein in accordance with the predefined protocol, the pulses are applied to the radial electrodes on different ones of the spines in a predefined sequence in which the pulses are not applied in immediate succession to the radial electrodes on mutually adjacent spines.
 16. The apparatus according to claim 11, wherein the radial electrodes comprise at least first and second radial electrodes on each of the spines, and wherein in accordance with the predefined protocol, the pulses are applied to only one of the first and second radial electrodes on each of the spines.
 17. A method for medical treatment, comprising: providing a basket assembly for insertion into a body cavity of a patient, the basket assembly comprising a plurality of resilient spines, which are configured to bow radially outward around a longitudinal axis of the basket assembly and are conjoined at a distal end of the basket assembly, and a plurality of electrodes, which are configured to contact tissue in the body cavity and comprise radial electrodes disposed on the spines and an axial electrode disposed on the longitudinal axis of the basket assembly; and applying to the electrodes, including the axial electrode, pulses having an amplitude sufficient to cause irreversible electroporation (IRE) in the tissue contacted by the electrodes.
 18. The method according to claim 17, wherein the spines have respective proximal and distal tips, wherein the proximal tips of the spines are coupled together at a at a proximal end of the basket assembly, and the distal tips of the spines are coupled together at a at a distal end of the basket assembly, and the spines bow radially outward when the basket assembly is deployed in the body cavity, whereby the radial electrodes contact the tissue in the body cavity.
 19. The method according to claim 18, wherein the basket assembly comprises a stable collapsed state, and wherein providing the basket assembly comprises a attaching a puller to the distal end of the basket assembly and slidably disposed within the insertion tube, so that the spines bow radially outward in response to pulling the puller in a proximal direction through the insertion tube.
 20. The method according to claim 17, wherein the insertion tube comprises a flexible catheter configured for insertion into a chamber of a heart of the patient, and the electrodes are configured to contact and apply the electrical signals to myocardial tissue within the chamber.
 21. The method according to claim 17, wherein the radial electrodes comprise a single respective radial electrode on each of the spines.
 22. The method according to claim 17, wherein the radial electrodes comprise multiple radial electrodes on each of the spines in respective locations that are longitudinally staggered among the spines.
 23. The method according to claim 17, wherein applying the pulses comprises applying a sequence of the pulses having an amplitude of at least approximately 200 V, and wherein a duration of each of the pulses is less than approximately 20 μs.
 24. The method according to claim 23, wherein the sequence of the pulses comprises biphasic pairs of the pulses, wherein each pair comprises a positive pulse and a negative pulse.
 25. The method according to claim 17, wherein applying the pulses comprises applying the pulses in a bipolar mode between one or more of the radial electrodes and the axial electrode.
 26. The method according to claim 17, wherein applying the pulses comprises applying the pulses in a unipolar mode between one or more of the electrodes on the basket assembly, including the axial electrode, and a common electrode that is separate from the probe.
 27. The method according to claim 17, wherein applying the pulses comprises applying the pulses between different pairs of the electrodes sequentially in accordance with a predefined protocol.
 28. The method according to claim 27, wherein in accordance with the predefined protocol, the pulses are applied in a bipolar mode between each of the radial electrodes and the axial electrode.
 29. The method according to claim 28, wherein in accordance with the predefined protocol, the pulses are applied simultaneously between two or more of the radial electrodes and the axial electrode.
 30. The method according to claim 27, wherein in accordance with the predefined protocol, the pulses are applied in a bipolar mode between multiple pairs of the radial electrodes, each pair including a first radial electrode on a first one of the spines and a second radial electrode on a second of the spines, which is not adjacent to the first one or the spines.
 31. The method according to claim 27, wherein in accordance with the predefined protocol, the pulses are applied to the radial electrodes on different ones of the spines in a predefined sequence in which the pulses are not applied in immediate succession to the radial electrodes on mutually adjacent spines.
 32. The method according to claim 27, wherein the radial electrodes comprise at least first and second radial electrodes on each of the spines, and wherein in accordance with the predefined protocol, the pulses are applied to only one of the first and second radial electrodes on each of the spines. 